It's a pity that HIALS asks a reasonable question, about something outside his personal experience, and the reception his post evoked.
Following is a report of an inadvertant ash cloud encounter by a NASA instrumented DC-8. Note the engine damage resulting from this seven minute encounter. The report is lengthy, but a short lead in,
On the flight to Sweden, in a moonless and cloudless sky at 0508 GMT on February 28, 2000, scientists onboard the DC-8 monitoring sensitive research instruments reported a sudden increase in measurements that indicated the presence of a volcanic ash cloud. showing aerosol data for the seven-minute encounter. This encounter was more than 200 mi north of the predicted maximum northerly extent of the plume and approximately 800 nmi from the volcano. The volcanic plume was about 35 hr old at this time.
The flight crew noted no change in cockpit readings, no St. Elmo’s fire, no odor or smoke, and no change in engine instruments. They did notice that no stars were visible, but this is typical of flight through high cirrus clouds.
After seven minutes the crew noticed that the stars had reappeared, and at about this time the scientists reported that the research instrument readings had returned to normal. There was still no change in engine or airplane instrument readings. The DC-8 crew made an airborne encounter report to the appropriate oceanic control agency.
In this ash encounter, the crew verified that there was no change in engine instruments. As such, they did not reduce engine power nor attempt to exit the cloud. This was because of the complete lack of indication of a volcanic plume, other than the sensitive scientific instruments, and because the crew was not aware of the recommendation to reduce power to idle. In addition, over the polar ocean at night, using visual flight rules in a “Non-Radar Environment,” it was probably not prudent to reduce power and descend even if the crew had been aware of the recommended procedure.
http://www.alpa.org/portals/alpa/vol...8AshDamage.pdf